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Archaeometric Investigations on Red Pigments

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Archaeometric Investigations on Red Pigments Dottorato di ricerca: Archaeometric Scienze della Terra investigations on red pigments: the Curriculum: Scienze Applicate alla provenance of Protezione dell’Ambiente cinnabar and the e dei Beni Culturali discrimination of synthetic and Dipartimento di Scienze della Terra natural ochres Indagini archeometriche su pigmenti rossi: la Coordinatore: provenienza del cinabro e G. B. ANDREOZZI la discriminazione tra ocre naturali e di sintesi Tutori: Adriana MARAS Davide BLEINER Michela BOTTICELLI XXVIII ciclo Abstract: Le indagini di provenienza possono fornire un contributo essenziale sia all’archeologo che al conservation scientist . La comprensione dei processi I Michela Botticelli Archaeometric investigations on red pigments produttivi in pittura, a partire dalla fase estrattiva in miniera, contribuisce contemporaneamente alla definizione della tecnica esecutiva ma anche delle rotte commerciali, col fine ultimo di apprezzare il valore artistico e storico di un’opera d’arte. Inoltre, le indagini di provenienza possono essere utili nella determinazione dei falsi. Fino a questo momento, le metodologie diagnostiche più comuni nelle indagini di provenienza hanno previsto la determinazione di isotopi o elementi in tracce. Nel caso specifico del cinabro, che è oggetto di questo lavoro, gli isotopi dello zolfo sono più comunemente impiegati nello studio di questo pigmento, per quanto esistano anche studi preliminari basati sugli isotopi di piombo e mercurio circoscritti a campioni da cava e non specificatamente a scopo archeometrico. Tuttavia, queste tecniche non hanno, finora, restituito la possibilità di distinguere il cinabro proveniente da depositi differenti. Nel caso in cui alcune miniere siano discriminabili, tale risultato viene nella maggior parte dei casi ottenuto a mezzo di tecniche dispendiose, economicamente e temporalmente. Inoltre, sebbene siano note dall’VIII secolo d.C. ricette per la produzione di un analogo pigmento sintetico, non esiste, ad oggi la possibilità di stabilire quando il naturale sia preferito al sintetico nella produzione artistica antica e il grado di diffusione di queste stesse ricette orientali nel mondo occidentale. In quest’ottica, il primo obiettivo della presente ricerca è di definire una metodologia sperimentale alternativa per la determinazione della provenienza del cinabro e della sua discriminazione rispetto ai prodotti di sintesi. Peso maggiore è stato dato a tecniche facilmente reperibili, micro o non-distruttive e a basso costo, criteri preferibili II nel settore dei Beni Culturali, vale a dire nello studio di opere con carattere di unicità. La seconda parte di questo lavoro è invece focalizzata sullo studio di ocre rosse, al fine di valutare una prassi investigativa che restituisca la possibilità di discriminare il pigmento naturale dal suo equivalente sintetico. L’uso di un prodotto sintetico, ottenuto dal riscaldamento di ocra gialla, è probabilmente noto fin dalla Preistoria. Differenti tecniche diagnostiche (microscopio elettronico a trasmissione, spettroscopia infrarossa e diffrazione a raggi-X) sono state impiegate fino ad oggi allo stesso scopo, per quanto un protocollo sistematico non sia stato ancora definito e permangano dubbi sulla discriminazione naturale/sintetico. Il presente elaborato si compone, dunque, di due sezioni: 1. Indagini archeometriche sul cinabro; 2. Indagini archeometriche sull’ocra rossa. La sezione 1 è stata prevista per valutare, per la prima volta, l’efficienza della diffrazione su polveri (XRPD) e della spettroscopia Raman negli studi di provenienza sul cinabro. Tale obiettivo è stato perseguito analizzando campioni di cinabro a provenienza nota concessi da diversi Musei di Mineralogia e Scienze Naturali italiani. L’analisi dei diffrattogrammi è stata seguita da affinamento Rietveld, al fine di individuare variazioni dei parametri strutturali eventualmente connesse ad una diversa provenienza. Gli stessi campioni sono stati analizzati anche attraverso spettroscopia µ-Raman, al fine di evidenziare variazioni composizionali legate ad una diversa provenienza. Allo stesso scopo, la spettrometria di massa a plasma accoppiato induttivamente (ICP-MS) è stata impiegata su un set ristretto di campioni, per determinare una prima lista di elementi in tracce che III Michela Botticelli Archaeometric investigations on red pigments possono fungere da markers per la provenienza del cinabro, non essendo presenti in letteratura studi precedenti al riguardo. Una volta stabiliti gli elementi di interesse, la loro concentrazione è stata misurata a più alta risoluzione mediante SF ( sector field )-ICP-MS. L’analisi diffrattometrica ha mostrato che i campioni provenienti dalla Cina possono essere discriminati sulla base del volume e dello strain residuo della cella cristallina, essendo questi parametri notevolmente superiori alla media per questa località. Questo dato è stato confermato dalla spettroscopia Raman. In particolare, il trattamento statistico dei dati spettroscopici ha messo in evidenza i parametri che contribuiscono a fornire informazioni archeometriche: una banda Raman addizionale nei campioni cinesi, attribuita al selenio come sostituente dello zolfo, ha confermato la loro discriminazione. La stessa tecnica analitica ha anche consentito inoltre di distinguere l’unico campione sintetico dai naturali. L’analisi statistica delle concentrazioni degli elementi in traccia ottenute in ICP-MS ha evidenziato quali elementi possono essere considerati come markers di provenienza. Il rame sembra essere un buon discriminante per i campioni di Almadén, la principale risorsa estrattiva europea. Il selenio si conferma come un elemento fortemente caratterizzante per i campioni Cinesi. Di fatto, appare possibile caratterizzare l’impiego di cinabro proveniente da miniere cinesi, in modo rapido e non- distruttivo. Questa informazione potrebbe rivelarsi di grande interesse sia per il riconoscimento di falsi, quando l’approvvigionamento da risorse orientali può essere escluso su base storica, che nella definizione delle rotte commerciali dal Medio Oriente all’Europa. Non è da IV sottovalutare anche la discriminazione del cinabro sintetico, che appare evidente dai dati spettroscopici. La sezione 2 ha per oggetto l’ocra rossa, in campioni naturali e sintetici. Questi ultimi sono stati ottenuti in laboratorio a partire da campioni di ocra gialla riscaldati secondo due procedimenti differenti. Anche un campione di ocra gialla è stato sintetizzato in laboratorio, secondo una procedura comunemente descritta in letteratura per la produzione del cosiddetto “giallo di Marte”. Il riscaldamento di questo campione con le stesse modalità delle ocre naturali ha permesso di ottenere due “rossi di Marte”, anch’essi oggetto della presente indagine. I campioni sono stati preventivamente caratterizzati in XRPD. La stessa tecnica è stata anche impiegata su un quantitativo minimo degli stessi campioni e seguita dall’affinamento Rietveld al fine di valutare variazioni strutturali indotte dalla sintesi che possano agire da markers nella discriminazione del prodotto di sintesi dal naturale. In particolare, la valutazione della dimensione dei cristalliti sembra essere promettente nella discriminazione sia dei gialli che dei rossi. Anche la morfologia dei campioni è stata investigata mediante microscopio elettronico a scansione (SEM), al fine di mettere in luce differenze nella forma dei cristalli di sintesi. L’acquisizione delle immagini da elettroni secondari (SE) ha evidenziato che il giallo di Marte è chiaramente discriminabile per morfologia dai suoi equivalenti naturali. Infatti, nel prodotto di sintesi i cristalli di goethite si sviluppano in forma acicolare, chiaramente distinguibili al livello di ingrandimento raggiungibile con la strumentazione impiegata e in V Michela Botticelli Archaeometric investigations on red pigments contrasto con la forma arrotondata che caratterizza tutti i campioni di ocra gialla naturale analizzati. Contemporaneamente, gli effetti composizionali indotti dalla sintesi sono stati valutati mediante spettroscopia infrarossa. Questa tecnica diagnostica si è rivelata sensibile nella differenziazione del giallo di Marte dalle ocre gialli naturali sulla base della valutazione delle bande attribuite ai gruppi ossidrilici. Al contempo, l’indagine in µ-FTIR è risultata essere la più efficace nella discriminazione dei rossi di sintesi. In particolare, la valutazione quantitativa delle bande caratteristiche degli ossidi e idrossidi di ferro ha consentito di individuare, in via preliminare, quali parametri possono incidere sulla discriminazione dalle ocre naturali, apportando un contributo significativo anche nella separazione dei prodotti ottenuti mediante procedure di sintesi differenti. VII Michela Botticelli Archaeometric investigations on red pigments VIII Alle donne e ai s tantivi fti della mia famiglia: a mia madre e alle mie nonne, alla resistenza al sriso e al cale familiare. IX Michela Botticelli Archaeometric investigations on red pigments Acknoedgements Here we are: time for gratitude. I would first like to thank my thesis advisors, Prof. Adriana Maras, of the Department of Earth Sciences at “Sapienza” University, for the support of my PhD study and related research, for her patience, motivation and suggestions. Her guidance helped me in all the time of research and writing of this thesis. I would also like to thank my second advisor, Prof. Davide Bleiner, Head of the Laboratory for Advanced Analytical Technologies at the EMPA Institute of Dübendorf, Switzerland, for his
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